1. Field
The disclosed concept pertains generally to network protectors and, more particularly, to network protectors carried and movable by a racking mechanism. The disclosed concept further pertains to network protector systems. The disclosed concept also pertains to automatic racking systems for electrical switching apparatus.
2. Background Information
Circuit breakers for medium voltage switchgear apparatus applications are generally housed in metal enclosures and are removable. The apparatus typically includes at least one levering-in mechanism or device to move a circuit breaker between a disconnect position, in which the primary contacts of the circuit breaker are fully disengaged from the mating primary contacts within the enclosure, and a connect position, in which the primary contacts of the circuit breaker and enclosure are fully engaged. Levering-in and levering-out/withdrawing of a circuit breaker can be accomplished by using a detachable hand crank, which is inserted into the levering-in mechanism to move the circuit breaker within its cell as a function of turning the crank.
Typically, power is provided to auxiliary devices and control circuitry through mating secondary contacts mounted with the circuit breaker in the enclosure. At some point during movement of the circuit breaker from the disconnect position to the connect position, the respective secondary contacts must be engaged in order that power is provided to the auxiliary devices and the control circuitry. When the secondary contacts are engaged, but the primary contacts are disengaged or disconnected, the auxiliary functions of the circuit breaker can be safely tested since the circuit breaker is not energized.
Low voltage secondary power distribution networks consist of interlaced loops or grids supplied by two or more sources of power, in order that the loss of any one source will not result in an interruption of power. Such networks provide the highest possible level of reliability with conventional power distribution and are, normally, used to supply high-density load areas, such as a section of a city, a large building or an industrial site.
Each source supplying the network is typically a medium voltage feeder system including a switch, a voltage reducing transformer and a network protector. As is well-known, a network protector is an apparatus used to control the flow of electrical power to a distribution network. The network protector includes a low voltage circuit breaker and a control relay which opens the circuit to the transformer upon detection of abnormal current flow. Specifically, the control relay typically senses the network voltages, the line currents and the phasing voltage, and executes algorithms to initiate circuit breaker tripping or re-closing actions. Trip determination is based on detecting reverse power flow, that is, power flow from the network to the primary feeder. Examples of network protector relays are disclosed in U.S. Pat. Nos. 3,947,728; 5,822,165; 5,844,781; and 6,504,693, which are incorporated by reference herein.
A network system is a redundant power delivery system including a plurality of primary feeders and associated network protectors. The transformer secondaries are electrically tied together, which increases the available fault current.
Network protectors are typically used in the enclosed spaces of underground vaults. Since about 1922, network protectors have been installed in underground concrete vaults in major city centers. Since that time, such network protectors were designed as a switch that was bolted in place and required manual removal from the corresponding enclosure. The relatively extreme environment of a network protector demanded special components. As a result, standard power circuit breakers were not utilized. In about 1999, the assignee of the disclosed concept developed a network protector with a power circuit breaker suitable for that environment. This power circuit breaker included a relatively high temperature composite housing and a robust mechanism for operation. Subsequently, the assignee's power circuit breaker included a removable, four position, draw-out power circuit breaker as part of the network protector. However, actuation of the draw-out mechanism requires direct user involvement in the environment of the network protector vault. Therefore, it is believed that known network protectors require a worker to manually draw-out or manually unbolt and physically remove the network protector.
However, as a result, the worker can be exposed to arc flash hazards, which can cause equipment damage, serious bodily injury or even death if done improperly. A worker performing network protector manipulation is in physical proximity with the network protector. Therefore, the workers are typically required to wear approved personal protection equipment (PPE) to resist serious injury or death that could result if an electrical failure were to occur during racking/draw-out/draw-in operations. However, PPE is generally bulky, hot and uncomfortable, which dissuades workers from wearing it.
U.S. Pat. No. 4,017,698, which is expressly incorporated by reference herein, discloses an automatic circuit breaker in a draw-out unit removably mounted within an enclosure. A levering mechanism is provided to manually lever out the draw-out unit on rails to permit complete disengagement of the circuit breaker from load and line terminals mounted within the enclosure without requiring unbolting operations. The levering mechanism is employed to operate the draw-out unit between engaged and disengaged positions. The levering mechanism comprises a mounting bracket welded to a channel member of a main support frame. A drive shaft including a worm gear extends through the mounting bracket in a direction perpendicular to the channel member. One end of the drive shaft is threaded into a square traveling nut which is located by a square aperture in the rear of the mounting bracket. The end of the drive shaft opposite the threaded end includes a pin which can be engaged by a cooperating socket at the end of a manual operating crank. A levering shaft extends through two side support plates and through the mounting bracket in a direction perpendicular to the drive shaft. Mounted upon the levering shaft within the mounting bracket is a main gear which is engaged by a worm gear. Also, mounted upon the levering shaft is an interlock cam which cooperates with a shutter pivotally secured to the mounting bracket to provide a safety interlock for the operating crank. At each end of the levering shaft is an engaging lever containing a roller. The levers and rollers cooperate with “J” shaped hooks mounted upon the enclosure. A spring-loaded pivot plate is mounted at the top of the mounting bracket and serves to prevent the raising of the shutter unless lifted by a protective barrier, or otherwise displaced.
In order to operate the draw-out unit from a disengaged fully rolled out position to a fully engaged position, the draw-out unit is manually rolled back along the rails into the enclosure until the rollers contact the rear edge of the J shaped hooks. In this position, disconnect structures are still physically separated from corresponding terminals. The shutter is then raised to allow insertion of the operating crank and engagement of the drive shaft. The crank is then manually operated to cause counterclockwise rotation of the drive shaft. The attached worm gear also rotates in a counterclockwise direction to cause corresponding counterclockwise rotation of the levering shaft and levers. The rollers will move downward into the slot of the J shaped hooks, pulling the draw-out unit into the enclosure. Continued manual rotation of the operating crank will cause continued rotation of the levers, pulling the draw-out unit into a completely engaged position within the enclosure. In this position, the disconnect structures engage the corresponding terminals. As the crank is manually rotated, the traveling nut is constrained by the square aperture in the mounting bracket and is drawn inward along the threaded end of the drive shaft. When the draw-out unit reaches the fully engaged position, the threaded end of the drive shaft contacts the bottom of the traveling nut, effectively preventing further rotation of the crank and drive shaft.
U.S. Pat. No. 6,897,388 discloses a portable circuit breaker racking apparatus including a housing having a base and a frame extending upwardly from the base. A wheel structure is coupled to the base to permit wheeled movement of the housing. A motor mount structure is coupled to the frame for generally vertical movement toward and away from the base. An electric motor, preferably a gear motor, is fixedly mounted to the motor mount structure. The motor has a rotatable shaft and an adaptor structure is operatively associated with the shaft. The adaptor structure is coupled directly to the shaft and is constructed and arranged to be coupled to a circuit breaker. A digital encoder is associated with the motor to track a position of the shaft of the motor and thus the position of the circuit breaker when coupled to the adaptor structure. Due to different circuit breaker designs having different elevation locations of racking, the apparatus includes an elevation adjustment feature.
The motor is controlled by a programmable logic controller (PLC). A cable electrically connects the motor with the controller. Operation of the PLC is achieved via an operator control station that communicates with the PLC at a cable to operate the PLC via a location remote from the circuit breaker when coupled with the apparatus. The cable has a length of about 40 feet to ensure that the operator can be a safe distance (e.g., per NFPA 70E) from the circuit breaker when moving the circuit breaker. Instead of using the cable, wireless communication can be employed between the control station and the PLC. The control station includes “raise” and “lower” buttons to control the motor. The position of the circuit breaker, coupled to the apparatus via an adaptor structure, is tracked by a digital encoder, preferably located on a fan portion of shaft of the gear motor. As the gear motor turns, the encoder sends ten pulses per revolution of the motor to the PLC. The encoder and the PLC allow the creation of a linear counter. As the motor rotates, it rotates the racking mechanism of the circuit breaker to cause the circuit breaker to move forward or backward in its cell housing. Therefore, for each rotation of the motor, the circuit breaker moves in or out a certain number of inches.
A “torque profile” protection utilizes the linear counter to provide an accurate location of the breaker during the racking process. A variable frequency drive is used to provide current or torque feedback from the motor to the PLC on a continuous basis. A torque profile generator program of the PLC generates an initial torque profile for each circuit breaker by establishing profile position points along the travel distance. Typically, 100 equal increments or number of encoder pulses are used. Using the linear counter, the program identifies the profile position point, checks the current or torque value and stores the information in the PLC memory for that particular circuit breaker. The PLC checks the variable frequency drive and stores the gear motor current value that corresponds to the breaker location or encoder pulse count. The gear motor current is a direct representation of torque. As the breaker is moved, the linear counter changes. At the next profile position point, the current or torque is checked and stored. This is done approximately 100 times between the positions of the breaker.
Once the torque profile of a breaker is generated and stored in memory, it is used as a reference for any future operations of the circuit breaker. The same profile position points used in generating the torque profile are used in monitoring the torque profile. In monitoring the torque profile, the linear counter is used to determine when the breaker is at a profile position point. When the breaker reaches a profile position point, the present motor current or torque is compared to the motor current or torque stored as a reference or as a base line. If the present value is larger than the stored value, the program initiates an over torque stop of the system.
The use of digital encoder positioning, current monitoring, and programmable logic control allow for program generation of a torque profile for each classification and/or type of medium voltage circuit breaker. The combination of the torque profile, position sensing of the circuit breaker and current monitoring give exact, accurate and extremely fast monitoring and protection of the circuit breaker and its cell housing. The apparatus advantageously knows the position of the circuit breaker at and between the connected and disconnected positions thereof at all times, without having to add some device in each breaker switchgear cell. The apparatus eliminates the need for operators to be within the “flash boundary” as defined by NFPA 70E and the requirement to wear specified personal protective equipment. The apparatus enhances operator safety and maintains precise monitoring and protection of the breakers and cell housings.